Low cost high density rectifier matrix memory

ABSTRACT

A high density memory device is fabricated three dimensionally in layers. To keep points of failure low, address decoding circuits are included within each layer so that, in addition to power and data lines, only the address signal lines need be interconnected between the layers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/780,909, filed Jul. 20, 2007, which is a continuation of U.S. patentapplication Ser. No. 11/229,941, filed Sep. 19, 2005, now U.S. Pat. No.7,460,384, which is a division of U.S. patent application Ser. No.09/887,201, filed on Jun. 22, 2001, now U.S. Pat. No. 6,956,757, whichclaims the benefit of and priority to U.S. Provisional PatentApplication Ser. No. 60/213,419, filed on Jun. 22, 2000, the entiredisclosure of each application being hereby incorporated by reference.

CROSS-REFERENCE TO RELATED PATENTS

This application makes references to U.S. Pat. No. 5,889,694 for a“Dual-Addressed Rectifier Storage Device” issued March 1999, to U.S.Pat. No. 3,245,051 for “Information Storage Matrices” issued April 1966,and those patents are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor storage devices, and moreparticularly to low cost memory resulting from manufacturingefficiencies which construct the devices in three-dimensions and whichincorporate efficient testing mechanisms.

2. Description of Prior Art

Many versions of three-dimensional memory arrays have been disclosed inthe prior art and they might be classified as being one of twotypes—stacked chips and layered manufacture.

Layered approaches are discussed in U.S. Pat. Nos. 4,525,921, 4,646,128,and 5,432,729 wherein Carson disclosed a technique for bonding two ormore memory chips one on top another to form a three-dimensional memoryarray. There have been variations on this design, such as by Kato inU.S. Pat. No. 5,051,865, in which he discloses an enhancement comprisingheat sink layers which are bonded between memory circuit layers to helpreduce overheating.

Of greater interest to the present invention, Zhang, in his U.S. Pat.No. 5,835,396, discloses an approach for manufacturing a layered memorydevice based on diode storage devices at the intersections of the rowsand columns within each layer and Rosner, in his U.S. Pat. No.4,442,507, discloses an electrically programmable read-only memory. Bothof these devices are manufactured upon a semiconductor substrate havingdecoding logic for the associated bit lines of the memory layers.

However, all of these devices have shortcomings. The bonding of multiplechips creates many points of failure and has high associated assemblycosts. Manufacturing devices in layers as disclosed in the prior art hasthe high cost of the base substrate and its electronic circuitry(manufactured using traditional semiconductor manufacturing means) andits associated complexity as well as the many potential points offailure of interconnecting the vast number of row and column bit linesof the various layers.

What is needed is a three-dimensional memory device which can retain theadvantage of lower cost manufacturing by not requiring a base layercomprising the bit line decoding circuitry and the high reliability offew layer interconnects. The present invention accomplishes this byincluding the decoding circuitry on each memory layer therebyeliminating the need for a base layer comprising bit line decodingcircuitry and thereby dramatically reducing the number of layerinterconnects to just power and a few address and data lines.

SUMMARY OF THE INVENTION

As advances continue to be made in the area of high densitysemiconductor storage, the need to keep the cost of these devices low iscritical for many applications. Many storage solutions, such as FLASHmemory (a non-volatile, rewritable memory technology), are currentlyvery expensive. As a result, many users of FLASH memory must upload anddownload the contents of their memory to other storage means, such as acomputer with a hard disk. This is because FLASH is so expensive thatrather than have many FLASH memory devices, users will typically ownvery few FLASH memory devices which they will reuse depending on theircurrent needs.

The present invention is a means for constructing a high densityrectifier matrices (as might be used to construct electronic memory)such that the costs are kept low. By constructing memory devices inthree dimensions and integrating simple test circuitry that can rapidlyverify the operation of such devices, storage densities can be made veryhigh while keeping testing time (and the associated costs of testing)very low. It is believed that memory devices can be manufactured whichwill be dramatically lower cost than current technologies.

The present invention also offers advantages over existing technology inthat this memory technology could be added to a variety of existingsemiconductor devices. Also, a one time programmable variation of thedevice could be created with little additional effort.

Finally, the present invention also offers advantages over existingtechnology in that this memory technology, by virtue of itsexceptionally simple design, will retain its cost savings even whenmanufactured three-dimensionally in layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Illustrates a prepared substrate for the present invention.

FIG. 2. Illustrates a front and side view after etching rows.

FIG. 3. Illustrates a front and side view of the rows following apassivation step.

FIG. 4. Illustrates a front and side view of the passivated rowsfollowing etching to open contacts through the passivation layer.

FIG. 5. Illustrates a front and side view of the device following finalmetalization and etching to create columns.

FIG. 6. Illustrates a second layer of the device having beenmanufactured upon a first layer of the device.

FIG. 7. Illustrates an interconnection pattern for quick testing of thedevice.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is a means for fabricating semiconductor memory inthree dimensions. This memory will be fabricated using a process similarto that used to construct Thin Film Transistor (TFT) flat paneldisplays—in particular, using deposited semiconductor material oninsulating substrates.

Refer now to the figures which show a preferred embodiment of theinvention. FIG. 1 shows a possible configuration of a preparedsubstrate. This substrate consists of a base (1) that is typically madeof glass, of Silicon Dioxide on a silicon wafer, or of other insulatingmaterial. On top of this is deposited about 1000 Å of metal (2) such asTiAu (Ti-gold), Ni (Nickel), or Al (Aluminum). Next is deposited 250 Åof N+ amorphous Silicon (N+δSi:H) (3) followed by 1000 Å of δSi:H (4).Finally, 100 Å of metal (5) is deposited. It should be noted that themetals and the thicknesses are not critical, so long as certainfunctional requirements are met. Bottom metal (2) can be made of anymaterial that will form an ohmic contact with the layer (3) above it.Bottom metal (2) should be thick enough to carry enough current toprovide the operational speed desired given any circuit capacitances.Layer (4) can be one of many materials that will form an ohmicconnection between layers (2) and (4) and need only be thick enough toprevent layers (2) and (4) from interacting in a non-linear way (i.e.,not a rectifying contact). Layer (4) can be one of many materials thatwill form a rectifying contact with top metal layer (5) and need only bethick enough to allow for an adequate junction depth for properoperation of that rectifying contact. Top metal (5) can be made of anymaterial that will form a rectifying contact with the layer (4) below itand need not be very thick because additional metal will be depositeddirectly on top of this in a later step. Finally all thicknesses must bethin enough to not create unacceptable aspect ratios (i.e., very tall,thin rows that are too tall for their widths), given the design featuresize, when these layers are etched as described below.

FIG. 2 shows the device following etching steps that result in manyparallel rows having many “towers” of rectifiers on each of them. Thewidth of these rows must be proportional to the thickness of thesubstrate layers so that the aspect ratio are manageable; if the rows ortowers are too tall, given their footprints, they will fall over. Thewidth of the channels between the rows and towers must be wide enough tominimize interaction of adjacent rows and columns.

FIG. 3 shows the rows following passivation—that is to say, the channelsbetween the rows and columns are filled in with insulating material (6)for structural strength, to provide a smooth surface for subsequentphotolithography steps, and to insulate the sides of the rows andcolumns from the deposition of the top metal layer for the columnconnections.

FIG. 4 shows the array following an etch to create openings to thetowers through insulating material (6) These openings are madeselectively in order to program the device according to U.S. Pat. No.5,889,694. Note that the thickness of the passivated material on top ofthose towers where no opening is etched must be sufficient to ensureelectrical insulation from the top metal layer (yet to be deposited). Itshould also be noted that all data bit connections could be opened inthe case of a programmable or writable device. Such a programmabledevice might have an additional layer or might otherwise provide for afusable or antifusable link in each of the towers used for data storage.This additional layer might also be made of a thin-film material that isa phase-change chalcogenide alloy similar to the film used to storeinformation on commercial CD-RW and DVD-RAM optical disks. Note thatsome of the etches go all the way down to the bottom metal layer (2) toenable the formation of direct connections to the bottom metal rows.Steps 3 and 4 can be partly combined by using a photosensitive spin-oninsulator, such as Polyimide, which can be deposited and thenphotolithographically patterned and developed.

FIG. 5 shows the final device following the deposition of top metallayer (7) and the etching of that layer into columns orthogonal to therows. The result of this process will result in a 2 dimensional array ofrectifying contacts (in this case, schottky diodes). The rows will beconnected by the top metal layer (7) and the columns will be connectedby the bottom metal layer (2). This top layer is expected to be about2000 Å thick, but should be thick enough to carry enough current toprovide the operational speed desired given any circuit capacitanceswithout creating any of the previously mentioned aspect ratio problemswhen the columns are etched.

It is believed that the simplest construction would be as describedabove utilizing schottky type rectifying contacts. However, many otherrectifying structures could be used; the bottom metal layer (2) and topmetal contact (5) could be ohmic and the junction between layers (3) and(4) could form the rectifying contact. For example, if layer (3) wasdoped N+ and layer (4) was doped P+, a P-N rectifying junction could beformed. Alternatively, some other semiconducting material, such as anorganic molecule, could be used in place of amorphous silicon. Anothervariation could include multiple rectifying junctions in series. Thismight be achieved by constructing a P-N junction and a schottkyjunction. With this multiple stacked rectifying towers design, if one ofthe rectifying devices was defective and formed an ohmic connectioninstead of a rectifying connection, the others in the series wouldresult in the tower still being a rectifying connection between the topcolumn and the bottom row as required for such a rectifier storagematrix.

FIG. 6 shows the result of this process if repeated on a substrate whichhas already had this process performed once. Note that the first layerwill most likely be planarized with a polishing step before depositingan insulating film to start the second layer. In the case of the DRSmemory constructed in multiple layers, these layers can be wired mostlyin parallel (as described in U.S. Pat. No. 5,889,694), only the bondingpads (9) would be interconnected by vias. As can be seen, the structureis identical except for the inclusion of vias (8) to interconnect thebonding points for power, ground and the complementary address inputs ofeach layer. The data bonding pads would be brought to the surface layerindependently so that each data bonding pad would connect to a singlelayer. Other configurations of wiring are possible. For example, if thedata lines of each layer were interconnected by vias and separation wasinstead accomplished by having some address lines connected to specificlayers so as to in effect enable only a single layer at a time andthereby multiplex the data connections. This would be the equivalent ofimplementing the selective powering of the device as is disclosed inU.S. Pat. No. 5,889,694 whereby each individually powered section of thedevice was comprised in a separate layer.

FIG. 7 shows an interconnection technique for the rows (and would berepeated for the columns) to enable the quick and inexpensive testing ofthe device. The most common faults expected to occur during thefabrication of the present invention are breaks in a line or shortsbetween two adjacent lines. With the present interconnection technique,all the alternate rows are connected end to end with a top layerconnection thereby creating two conductive snakes—one comprising all theeven numbered rows (10) and the other comprising all the odd numberedrows (11). By doing so, one can test the device by probing the two endsof each snake and checking for continuity. This will enableinstantaneously checking the all the rows for any breaks with just fourprobe points. Also, shorts between rows can be identified simultaneouslyby checking for any continuity between the two snakes. The columns wouldbe simultaneously checked in an identical fashion. Also, in the same wayand at the same time, the device can be checked for shorts between thetop columns and bottom rows (taking care to select one range of voltagelevels for the rows and another range of voltage levels for the columnssuch that the diodes are reverse biased). By combining this testtechnique with the multiple stacked rectifying towers design, if adevice passes this instantaneous test, one can be highly confident ofhaving a working device. After identifying any defective devices (which,if the devices are manufactured inexpensively enough, would most likelyjust be marked to be discarded), these top layer connections are etchedaway to separate the lines or the edges of the chips where theinterconnections are found could be cut back just enough to remove thoseedges and their interconnections.

This process is initiated by depositing layers on an insulatingsubstrate such as plastic or glass. But, it should be noted that thisinsulating substrate does not have to be limited to plastic or glass. Ashas been shown in the prior art, this process could be performed on topof a normally fabricated semiconductor device such as a memorycontroller chip, an error detection and correction chip, a datadecryption (and, optionally, encryption) chip (such as for security likeDES or PGP or for storage efficiency like MP3), or a microprocessor, orthe like that has been passivated with a top layer of protecting glassor other insulator. An array of LEDs or diode optical sensors might beconstructed upon an image processor to construct a micro display orcamera. In its simplest form, a diode matrix (such as was disclosed byRobb in his U.S. Pat. No. 3,245,051, issued in 1965) could beconstructed on top of a memory controller comprising the addressdecoding, the row and column selection logic, the linedriver-amplifiers, and the sense amplifiers; in this case, theindividual connections to the rows and columns would be made up from thecontroller substrate. Since a storage array could cover a greater areathen, say, a memory controller chip, multiple memory controllers couldbe placed on the chip to be covered according to the present inventionthereby providing redundant controller circuits to operate the storagearray, resulting in higher fabrication yields. Furthermore, this processcan be performed on top of itself resulting in a three-dimensionalconstruction of this memory device with many layers. One would only thensimply have to interconnect the two or more layers with vertical viaconnections. In the case of the DRS memory constructed in multiplelayers, these layers can be wired mostly in parallel; just as the DRSarray can be divided into multiple areas on one layer (as described inU.S. Pat. No. 5,889,694), these multiple areas could be constructedvertically with this technique.

Other variations would include the fabrication of integrated memorycircuits using other than traditional photolithographic semiconductormanufacturing techniques. By combining lower cost manufacturingtechniques with the higher densities resulting from three-dimensionalfabrication, the cost per bit can be driven even lower.

The foregoing description of an example of the preferred embodiment ofthe invention and the variations thereon have been presented for thepurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise forms disclosed.Many modifications and variations are possible in light of the aboveteaching. It is intended that the scope of the invention be limited notby this detailed description, but rather by any claims appended hereto.

1. An electronic memory device comprising: a substrate; and disposedabove the substrate, a plurality of layers of memory circuitry defininga memory array, each layer of memory circuitry comprising a plurality ofstorage locations, wherein the memory array is organized as a pluralityof individually enablable sub-arrays.
 2. The electronic memory device ofclaim 1, wherein, during operation, at least one of the sub-arrays isenabled and at least one of the sub-arrays is not enabled.
 3. Theelectronic memory device of claim 1, wherein each of the plurality oflayers corresponds to a sub-array.
 4. The electronic memory device ofclaim 1, wherein the substrate comprises at least one of addressdecoding logic, row and column selection logic, a line driver-amplifier,or a sense amplifier.
 5. The electronic memory device of claim 1,wherein the substrate comprises at least one of error detection andcorrection logic, data decryption logic, data encryption logic, datacompression logic, or data decompression logic.
 6. The electronic memorydevice of claim 1, wherein the substrate comprises a semiconductormaterial.
 7. The electronic memory device of claim 1, wherein eachsub-array is individually powered.
 8. The electronic memory device ofclaim 1, wherein the substrate consists essentially of an insulatingmaterial.
 9. The electronic memory device of claim 1, wherein thesubstrate comprises a plurality of memory controllers.